1. Field of the Invention
The present invention is concerned with an improved process for converting natural gas into a low-vapor-pressure liquid hydrocarbon mixture, which for instance can be more easily transported from a remote oil or gas field to a market. More particularly, this invention deals with a novel processing scheme to convert light gases (Methane to Butanes) into aromatics, C.sub.5 +hydrocarbons and a hydrogen-rich by-product gas. This process, if applied in a petroleum refinery, also allows to increase the yield of higher value liquid products (BTX) at the expense of less valuable gas products. This process may also be used to produce hydrogen from natural gas.
2. Description of Prior Art
Conversion of the C.sub.1 -C.sub.4 fractions of Natural Gas into higher molecular weight components is commonly achieved by first fractionating the feed into essentially pure components, such as Ethane or Propane and then converting each fraction individually into heavier, more valuable components such as olefins or aromatics, using either non catalytic thermal processes (e.g. steam pyrolysis or thermal cracking for the conversion of Ethane into Ethylene), or catalytic dehydrogenation processes or a combination of both types of processes in which an essentially pure lighter feed component, Ethane for example, is first converted into Ethylene by steam cracking at high temperature, and the Ethylene is separated and subsequently converted to aromatics catalytically at moderate temperature in a second step. The catalysts used are composed primarily of Silica (SiO.sub.2) in the form of Silicates or Alumino-Silicates. It is claimed that the acidic nature of the Silica-based catalysts (Bronsted acid sites) or the cage-like structure of the catalyst crystal (such as a ZSM-5 zeolite) are determining factors in these conversion processes. Some of these processes are oxidative in nature and result in the formation of oxidation products, such as water, which have no fuel value and must be separated from the hydrocarbon product. Still, other processes use "super acids" as catalysts, including various oxidative reagents (Fluorine, Chlorine, Sulfates . . . ). These processes also result into oxidation products which must be separated out for disposal or for reclaiming.
Some of the known catalytic processes for the conversion of lighter components C.sub.1, C.sub.2, of natural gas, mostly produce light olefins and LPG components which must then be separated out and converted to heavier components in a second step, using different catalysts.
The early commercial processes (Fischer-Tropsch) available to produce hydrocarbon liquids from gases did not use natural gas as a feed stock but various mixtures of CO and H.sub.2. These can be derived from natural gas by various oxidative processes, so that the conversion of natural gas into hydrocarbon liquids requires additional preprocessing and the use of several different catalysts. These processes based on Syngas (CO+2H.sub.2) also proceed through many intermediate steps in which oxygenated products (Methanol for instance) are formed and must be separated, purified and de-oxygenated into other intermediate chemical specie before reaching the final goal of a marketable liquid hydrocarbon mixture. The catalysts used in these processes generally contain large amounts of Silica. Thermal pyrolysis of Methane in an electric arc furnace has been used to produce acetylene, which is a much more stable component at very high temperatures than aromatics or naphta, in the absence of any catalyst. Although acetylene is an important raw material for organic synthesis, it is not easily transportable, and cannot be marketed together with crude oil or condensate as a liquid mixture.